THE TERTIARY FORMATION. 5 



The " lucid spots," or mark of the place of muscular attachment, afford an interest- 

 ing subject for examination and compai'ison. It is generally impossible to see them 

 fairly in the living carapace or in the uncleaned dead valves. Some fossil valves 

 present them clearly ; but mineralization frequently obscures them. To facilitate the 

 examination of the spots, it is necessary to boil the valves, both recent and fossil, in a 

 weak solution of potash,^ after which their structure is much more easily observed. 

 The boiling may be carried on in a flask over a spirit- or gas-lamp for ten minutes or 

 more, — as long as an hour, if found requisite. My friend Mr. W. K. Parker, who has 

 kindly favoured me with some of the finest and cleanest specimens in my collection, 

 informs me that the process above mentioned is appropriate and indeed necessary for 

 the preparation of Foraminifera and other microzoa for the cabinet. 



Baird, Fischer, Zenker, and Liljeborg have indicated the lucid spots in their 

 figures of Cyprides and Cytheres ; but frequently, owing to the partial opacity of the 

 valves in the recent state, and the difliculty of defining the spots externally, only 

 portions of the groups of spots are given ; and in some of the figures by Fischer and 

 Liljeborg the spots appear to have been sketched in without exact reference to their 

 position on the valve, since they are in these instances represented parallel with 

 the upper and lower borders of the valves, whereas the elongate spots are, as a rule, 

 obliquely situated. 



The lucid spots occupy a sub-oval space, or follow a short transverse linear sulcus, 

 near the middle of each valve, and rather towards the ventral border ; usually slightly 

 in advance of the centre, but sometimes behind it. The sub-oval space is sometimes 

 faintly raised externally ; the spots themselves, however, are almost always concave 

 externally and convex internally. 



Among the Cyprinoi three styles or systems of arrangement of the spots obtain. 

 These are illustrated by some of the figures in the accompanying Plates, and will serve 

 as terms of comparison. 



A. A system of about seven elongate oval spots, arranged in four unequal parallel 

 obhque rows ; the two outer (upper and lower) rows are each formed of two long 

 spots, frequently coalescent. The obliquity is directed in a line from about the antero- 

 dorsal angle, or the anterior hinge, towards the posterior third of the ventral margin. 

 (See Plate I, figs. 7 «, 7 ^, 7 c, 9 a.) 



B. A system of six sub-oval spots arranged in two transverse (vertical) rows ; 

 the front row consisting of four, and the hinder row of two spots ; the hinder spots are 



1 About 1 part oi liquor potassce with 19 parts of water (and more of the potash if required, according 

 to the coarseness and foulness of the specimens, up to 1 part in 4). 



My friend Mr. Harris, of Charing, also has shown me some of his Cretaceous Entomostraca, &c., 

 thoroughly cleanse by the action of water only, in a phial, half-filled with water, lashed to a small barrel 

 (externally fitted as a water-wheel, and propelled by a stream), in which he washes his Chalk-detritus. 



